This invention relates to cellular radio telephone systems, and more particularly to devices for extending the effective range of a cellular base station used in a digital cellular system, such as a Time Division Multiple Access (TDMA) cellular system, by repeating and frequency-translating one or more channels used by such a base station.
Cellular telephone systems impose limitations on antenna height and transmitter power of base stations to avoid interference among cells and to permit channels to be reused. Although such limitations reduce the effective size of the area which the base station may serve, a small cell size is desirable in those regions of the cellular system in which the density of cells is high.
However, in other regions of the system, it is desirable to enlarge the coverage area of a cell. For example, in regions of low cell density, such as at the periphery of a cellular system or along isolated rural stretches of highway, it may be desirable to provide cells covering a substantially greater area. It is often difficult to provide consistent coverage over a desired large geographic area from a single base station site. Although high antenna height and increased transmitter power may improve coverage, geographic features and other obstructions may obscure the radio signal path. In addition, it is difficult to limit the coverage of a cell to a desired predefined region when high antennas and powerful transmitters are used. One solution to this problem is to divide the desired coverage area into at least two cells and provide a complete additional cellular base station for each cell. However, cellular base station equipment is generally expensive and requires substantial physical space and environmental support.
Accordingly, several "cell-extender" devices have been developed for use in cellular systems for extending the effective coverage area of a cellular base station. A typical cell extender is used to bidirectionally repeat selected channels of a base station (referred to as a "donor cell") at a remote location to improve the communications quality between the donor cell and subscriber terminals over the quality which would be achieved if the donor cell were to communicate directly with subscriber terminals in the vicinity of the remote location. Such cell extenders may employ equipment which is less complex and less expensive than that of the donor cell. Such economy is achieved, at least in part, by operating substantially transparently with respect to the donor cell and the subscriber terminals, and allowing the donor cell to perform in its normal course of operation certain functions which otherwise might be performed by an independent cell site. Thus, the donor cell and the cell extender effectively "share" certain common facilities provided by the donor cell.
In prior art cell extenders designed for use in analog cellular systems, such as the AMPS cellular system used in North America, a cell extender may repeat and translate at least one of the donor cell's control channels and a plurality of the donor cell's voice channels.
A cell extender typically becomes involved in a conversation (i.e. acts as an intermediary between a donor cell and a subscriber terminal) in one of two ways. The subscriber unit may be engaged in an existing conversation, communicating directly with the donor cell, and may travel into the coverage area of the cell extender. Alternatively, the subscriber unit may be within the coverage area of the cell extender and may attempt to originate a call or answer a page while locked onto the cell extender's control channel.
In one prior art embodiment, when a subscriber unit involved in a conversation and communicating directly with a donor cell moves into the coverage area of the cell extender, the cell extender sends a hand-off message to the subscriber unit over the forward voice channel to instruct the subscriber unit to begin using a second voice channel (i.e., one assigned to the cell extender) instead of the voice channel assigned to the donor cell. Then, the cell extender acts as an intermediary by translating and repeating the donor cell voice channel on the second voice channel. The cell extender communicates with the donor cell over the channel originally used by the subscriber unit, and communicates with the mobile over the second voice channel.
Ordinarily, the timing of the repeated signals, and of most instructions to the subscriber unit, is not critical. For example, within a particular cell, a voice channel is used by a single subscriber unit at one time. When involved in a conversation, the subscriber unit is always receptive to hand-off instructions. In addition, since the voice channels are analog signals, the propagation delay through the cell extender's voice channel equipment, and any additional propagation delay introduced by the length of the RF path between the subscriber unit, cell extender, and donor cell, have little if any effect on the operation of the system.
In digital cellular systems, signal timing is more critical. For example, in Time Division Multiple Access (TDMA) systems, in which voice channels are shared by as many as six subscriber units, voice channel transmissions from each subscriber unit must actually arrive at the base station aligned within a predefined time slot to avoid interfering with the signals of other subscriber units. Accordingly, TDMA cellular systems have facilities for measuring the RF path propagation delay and require the subscriber unit to adjust the timing of its transmissions to compensate for such delays such that the transmissions arrive within the predefined time slot. However, the amount of timing adjustment provided by the subscriber unit in TDMA systems is limited. Because the RF path propagation delay cannot exceed that which the subscriber unit can compensate, the limited timing adjustment places a corresponding upper limit on the effective length of the RF path between the base station and the subscriber unit. In TDMA systems, this limit is approximately 92 km. Accordingly, subscriber terminals in TDMA systems may range a maximum of approximately 92 km from the base station.
Prior art cell extenders for use with digital cellular systems, such as TDMA, have been developed, and are largely similar to cell extenders for analog cellular systems. Prior art cell extenders of which we are aware have merely repeated the digital data streams of the TDMA voice channel. As a result, any additional propagation delays introduced by the cell extender equipment, and by the RF path between the cell extender and subscriber terminal, are visible to the base station, and effectively reduce the subscriber terminal-to-base station range.
Another critical timing concern in digital cellular systems is the timing alignment of a hand-off message as received by a subscriber unit. In TDMA systems, hand-off instruction messages are interposed in the forward digital traffic channel (FDTC) data stream received from the base station. In order for a subscriber unit communicating directly with a base station to properly receive a hand-off instruction from a cell extender, the hand-off message must actually arrive at the subscriber unit in precise alignment with the data stream from the base station.
The prior art cell extenders which have been constructed for use with digital cellular systems of which we are aware have not provided for proper alignment of hand-off messages as received by subscriber terminals.